Carbon fibers in ceramic cores for investment casting

ABSTRACT

A method of producing a ceramic core for investment casting is provided. The method includes injecting a slurry into a disposable die. The slurry includes ceramic particles, a binder, and carbon fibers. The method also includes a first heating to eliminate the disposable die, leaving a cured ceramic core comprising the ceramic particles, binder, and carbon fibers.

INTRODUCTION

The present disclosure generally relates to investment castingcomponents and methods for making them. In particular, the presentdisclosure relates to ceramic cores used to produce internal channelsinside turbine blades in the investment casting process.

BACKGROUND

Investment casting often utilizes cores to produce internal channelsinside cast metals. A molten metal or alloy is poured into a moldcontaining a core. After the metal solidifies, the core is removed toleave behind the internal channels. The architecture of the internalchannels is determined by the features of the core.

Cores formed through injection molding and other conventional processescan produce simple hollow channel architectures. However in someapplications, such as cast blades for gas turbines, more complex channelgeometries are desirable due to their improved blade performance, whereair is blown through the hollow channels of the cast blade for cooling.Improved blade cooling performance can take the form of reduced coolingair flow, which allows for increased utilization of air for combustionand thus increases engine thrust. Higher blade cooling performanceallows for an increase in combustor operating temperature and improvedthermodynamic efficiency, resulting in better specific fuel consumption,while still maintaining turbine blade component temperatures within anacceptable range for durability. Especially useful channel geometriesfor turbine blade cooling circuits are described in, for instance, U.S.Pat. No. 5,660,524, U.S. Pat. No. 6,036,441, U.S. Pat. No. 6,168,381,U.S. Pat. No. 6,595,748 and U.S. Pat. No. 6,832,889. A major limitationto commercial implementation of these representative cooling circuits inturbine blades is the inability to produce the necessary ceramic coresas single piece articles by conventional molding techniques. Examinationof the cooling circuit geometries of interest shows that there exists nosingle parting line allowing the construction of separable mold halvesto enable removal of a molded part without destroying part of the formedstructure. Accordingly, production of cores to produce suchsophisticated cooling circuits requires elaborate multi-step processeswhere the geometry is broken up into several moldable sections, eachwith its own separate mold tooling. The individual molded sections arethen assembled after molding and firing, with a concomitant reduction incore yield for precision investment casting, due to the loss of registrybetween the parts. The multi-piece process is also much more expensivedue to the capital cost of multiple mold tools, the hand work needed forcore finishing and assembly, and the further reduction in net castingyield, due to the poorer dimensional tolerances and mechanical stabilityduring casting of the multi-piece core assembly.

One way to produce cores with both simple and complex channelarchitectures is with disposable core dies (DCD) described in, forinstance, U.S. Pat. No. 7,487,819. The core is formed by injecting aslurry containing ceramic particles and an organic binder into adisposable core die. The slurry is then cured and then fired to producea solidified ceramic core. The disposable core die can be removedbefore, during or after the core firing process, for instance by achemical, thermal or mechanical process.

Ceramic core materials used in the investment casting industry are oftenmade predominately of silica (SiO₂). Silica is a commonly used corematerial in investment casting because of its low coefficient of thermalexpansion, high-temperature dimensional stability, and its ease ofremoval from the casting. Articles made by investment casting are castmetal or metal alloys. In some instances, this metal may react with theconventional silica-based ceramic core. Therefore, the use ofsilica-containing core materials for casting of reactive metals is knownto be problematic, as silica may react with certain metals during thecasting process.

Yttrium addition to an alloy is one approach for improving the oxidationresistance of nickel-based superalloys at the service temperature ofturbine airfoils. However, yttrium can react with silica during casting,leading to depletion of the yttrium in the alloy and introducingcomponents into the alloy that debit the mechanical properties and theability to further process turbine airfoils, resulting major limitationsin using silica cores for the casting of these (reactive) nickel-basedsuperalloys.

Alumina and yttria materials have been used in casting to reduce oreliminate this reactivity problem. Alumina, for instance, is lessreactive than silica. However, alumina is harder to process than silicamaterials with respect to higher temperatures required for processing.This introduces problems with dimensional tolerances because of thehigher coefficient of thermal expansion and inherent large creep raterelative to silica. Alumina cores can also require more extreme leachingconditions for their removal after casting because of their lowersolubility and/or leach rates. These constraints apply equally toceramic cores formed through conventional methods such as injectionmolding, as well as those formed by the DCD process.

Therefore, there is a need for a low reactivity material systemcompatible with DCD processing for producing cores that can generatecast articles with complex internal channel architectures made usingreactive metals and alloys.

SUMMARY

The following presents a simplified summary of one or more aspects ofthe present disclosure in order to provide a basic understanding of suchaspects. This summary is not an extensive overview of all contemplatedaspects and is intended to neither identify key or critical elements ofall aspects nor delineate the scope of any or all aspects. Its purposeis to present some concepts of one or more aspects in a simplified formas a prelude to the more detailed description that is presented later.

The foregoing and/or other aspects of the present invention may beachieved by a method of producing a ceramic core for investment casting.In one aspect, the method includes injecting a slurry into a disposabledie, the slurry including ceramic particles, a binder, and carbonfibers. The method also includes a first heating to eliminate thedisposable die, leaving a cured ceramic core including the ceramicparticles, binder, and carbon fibers.

The foregoing and/or aspects of the present invention may also beachieved by a method of making a cast metal component includingproviding a non-reactive ceramic core including ceramic particles, abinder, and carbon fibers to contact with a reactive metal to producethe cast metal component.

The foregoing and/or aspects of the present invention may also beachieved by a cured ceramic core including ceramic particles, a binder,and carbon fibers, the core having voids substantially throughout andgenerally aligned with an axis of the core.

Other features and aspects may be apparent from the following detaileddescription, the drawings, and the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an illustration of a thin wall disposable core die.

FIG. 2 is an illustration of a solidified ceramic core showing fibersoriented predominantly along the flow of the slurry, according to anembodiment of the present invention.

FIG. 3 shows a scanning electron microscopy (SEM) micrograph of alongitudinal view on a polished surface showing fibers orientingpredominantly along the flow of slurry.

FIG. 4 shows a SEM micrograph of a transverse view on a polished surfaceshowing extensive cracks (˜10 μm in width) leading to the part interior.

FIG. 5 XRD Spectrum of an as-fired Surface of a core fired up to 1650°C.

FIG. 6 illustrates an increase in the Modulus of Rupture at roomtemperature (RT MOR) with the addition of carbon fiber.

DETAILED DESCRIPTION

The detailed description set forth below in connection with the appendeddrawings is intended as a description of various configurations and isnot intended to represent the only configurations in which the conceptsdescribed herein may be practiced. The detailed description includesspecific details for the purpose of providing a thorough understandingof various concepts. However, it will be apparent to those skilled inthe art that these concepts may be practiced without these specificdetails. In some instances, well known components are shown in blockdiagram form in order to avoid obscuring such concepts.

Ceramic cores are used to produce internal channels inside turbineblades in the investment casting process. Blades made of Y-bearingmetals or alloys improve engine service temperatures by 50° C. The useof the silica-based cores for casting of these reactive metals or alloysposes melting issues, as silica reacts with certain metals during thecasting process. US 2017/0144216 assigned to General Electric Company,and hereby incorporated by reference, discloses a ceramic corecomprising alumina particles and siloxane binders; non-reactive coresthat reduce or eliminate the reactivity problem.

Especially, cores based on non-reactive Al₂O₃/mullite formulation needto possess significantly more porosity so that the core may be readilyremoved from the internal of a blade, and yet structurally strong enoughto survive the core manufacturing process. The present inventionprovides a method to produce core bodies with improved mechanicalstrength and increased porosity by introducing an optimum amount ofcarbon fibers to the non-reactive core fabrication process.

The non-reactive cores according to an aspect of the invention includesiloxane binders, fugitive species, and ceramic powders. The siloxanebinders and fugitive species, upon thermal decomposition, createporosity in the final core body. Both pre-existing and newly formedceramic phases give strength to a core body to maintain dimensionalintegrity. Further improvements on core and casting quality call forhigher mechanical strength. On the other hand, a porosity of more than40 vol % with an interconnected pore structure helps to increase theleaching efficiency during the core removal process.

To address the seemingly contradictory requirements, the presentinvention provides Al₂O₃-siloxane based slurry with added carbon fiberfor making ceramic cores using the DCD process. The carbon fibersstrengthen the core body during the manufacturing process and introduceadditional porosity in the finished core for the ease of core leaching.Thus, the present invention provides ceramic cores with unique featuresthat include open porosity, and increased mechanical strength at thesame time. Traditional metal dies or DCD may be used for this process,as may injection molding or any other type of conventional molding.

The present invention provides a ceramic core which has compatibilitywith the DCD process, is substantially non-reactive during casting, andallows for the production of structures with intricate internal shapeswhile possessing the compatibility with the alloy sought for articlesformed by investment casting.

As mentioned above, slurry may be transferred into a disposable coredie. Once the slurry is transferred into the die, the slurry may becured to form a solidified article, that is, for example, a green body.In some embodiments, the temperature for curing is between 25° C. and110° C. In other embodiments, the temperature for curing is between 40°C. and 80° C. In still other embodiments, the temperature for curing isbetween 50° C. and 70° C. In yet other embodiments, the temperature forcuring is between 55° C. and 65° C. In some embodiments, the time forcuring is between 1 hour and 24 hours. Any time and temperaturecombination that will cure the slurry to a green body is acceptable. Asa non-limiting example, the curing step may be approximately two hoursat 50° C. As another non-limiting example, the curing step may beapproximately 24 hours at 35° C. In another non-limiting example, thecuring may be for 6 hours at 55° C. The cured product includes asemi-hardened ceramic material including fibers encased in the DCD die.

In some embodiments, the conversion occurs in a two-step firing process.The first low-temperature firing step converts the silicon in thesiloxane binder to amorphous silica while not significantly degradingcarbon fibers in the core body. This “low fire” step, in someembodiments, burns off the die. In some embodiments, the first step isperformed for between 10 and 20 hours. In some embodiments, the firststep is performed for between 12 and 18 hours. In other embodiments, thefirst step is performed for approximately 16 hours. In some of theseembodiments, the first temperature is between 250° C. and 750° C., orthe first temperature is between 350° C. and 650° C. In otherembodiments, the first temperature is between 400° C. and 600° C., orthe first temperature is between 450° C. and 550° C.

The core then undergoes further heat treatment (i.e., a high-temperaturefiring). In this step, the amorphous silica in combination with theexcess alumina is converted to mullite and the carbon fibers are totallyburnt out, resulting in a desired microstructure with a predominantlyelongated pore structure. In some embodiments, the temperature for thisconversion is between 1050° C. and 1700° C., or between 1150° C. and1650° C., or between 1200° C. and 1650° C. In some embodiments, thetemperature for this conversion is between 1400° C. and 1650° C., or thetemperature for this step is between 1450° C. and 1650° C. In stillother embodiments, the temperature for this step is between 1500° C. and1650° C., or the temperature for this step is between 1500° C. and 1600°C. In some embodiments, the temperature for this conversion is between1550° C. and 1650° C., or the temperature for this conversion is between1575° C. and 1625° C. In other embodiments, the temperature for thisconversion is between 1400° C. and 1600° C., or the temperature for thisconversion is between 1450° C. and 1600° C. In yet other embodiments,the temperature for this conversion is between 1550° C. and 1600° C. Insome embodiments, the time utilized for this conversion is between onehour and 24 hours. In some embodiments, the time utilized for thisconversion is between one hour and 12 hours. In some embodiments, thetime utilized for this conversion is between one hour and 6 hours. Insome embodiments, the time utilized for this conversion is one hour. Anytime and temperature combination that will maximize the conversion ofthe amorphous silica-containing core to a mullite-containing ceramiccore as described herein is acceptable.

In some embodiments, there may be three separate heating events used forthis procedure.

In the first heating event, the slurry is cured to form a green body.The second heating event is the low fire step described above. The thirdheating event is the conversion of silica in the presence of excessalumina to mullite. In some embodiments, the curing and firing steps maybe accomplished in a single furnace in a single run. In otherembodiments, the firing steps may be accomplished in a single furnace ina single run.

The formed ceramic core comprises mullite, alumina, and free silica.Free silica may be present in the ceramic core, but much of this silicais encapsulated by mullite, rendering it “hidden” (or “substantiallynon-reactive”) from the metal, not available for reaction with metalalloy on a continuous basis, functionally inert, and compatible withcasting of reactive alloys. There may be some free silica present in theceramic core that is potentially reactive, but these amounts should beminimal. In some embodiments, the ceramic core contains up to 11 wt %free silica, or the ceramic core contains up to 10 wt % free silica, orthe ceramic core contains up to 8 wt % free silica. In some embodiments,the ceramic core contains up to 6 wt % free silica, or the ceramic corecontains up to 3 wt % free silica. In some embodiments, the ceramic corecontains up to 1 wt % free silica. In some embodiments, the ceramic corecontains between 0.1 wt % and 10 wt % free silica. In other embodiments,the ceramic core contains between 0.1 wt % and 5 wt % free silica, orbetween 0.1 wt % and 6 wt % free silica. In still other embodiments, theceramic core contains between 1 wt % and 10 wt % free silica. In someembodiments, the ceramic core contains between 1 wt % and 5 wt % freesilica, or between 0.1 wt % and 3 wt % free silica. In yet otherembodiments, the ceramic core contains between 1 wt % and 3 wt % freesilica, or the ceramic core contains between 0.5 wt % and 1 wt % freesilica. In some embodiments, the ceramic core contains between 2 wt %and 8 wt % free silica. In still other embodiments, the ceramic corecontains between 3 wt % and 6 wt % free silica. In other embodiments,the ceramic core contains between 0.1 wt % and 1 wt % free silica.

For purposes of this disclosure, the majority of the silicon-containingconstituents initially present in the slurry in the form of siloxanebinders are converted to the silicon-containing 3:2 mullite phase.However, it is to be understood that, in addition to 3:2 mullite andalumina, the thermally-converted slurry may also include othermaterials, such as silica, other Al₂O₃—SiO₂ polyforms that are below theXRD detection limit. The remaining free silica may be encapsulated bythe formed mullite, rendering it non- or minimally-reactive forsubsequent casting steps.

According to an aspect, the present invention incorporates carbon fiberinto the core formulations for producing ceramic cores.

FIG. 1 is an illustration of a thin wall disposable core die 100 thatcan be used to manufacture a turbine airfoil. A slurry comprisingceramic particles, a binder, and carbon fibers according to anembodiment of the present invention may be directionally added into theinterstices and channels of the thin wall disposable core die 100. Theslurry is then subjected to curing to form a cured ceramic core. Thedisposable core die 100 is removed to leave behind the cured ceramiccore as shown in FIG. 2. FIG. 2 is an illustration of the cured ceramiccore after being fired to form a solidified ceramic core 200. The carbonfibers 201 align in the direction of slurry flow as shown, for example,in FIG. 2.

FIG. 3 shows a scanning electron microscopy (SEM) micrograph of alongitudinal view on a polished surface showing carbon fibers orientingpredominantly along the flow of slurry. The sample was fired up to 500°C. According to an embodiment as shown in FIG. 3, during the slurryinjection molding process, carbon fibers orient predominantly along thedirection of slow flow. This carbon fiber alignment is critical inreducing cracks during the subsequent plastic die removing phase. Carbonfibers remain in the core body after thermal decomposition of plasticdie around 500° C. (first fire), reinforcing the core body becausecarbon fibers do not oxide until at a higher temperature.

During the subsequent second/setter fire up to 1650° C. in air, carbonfibers fully oxidize to CO and CO₂. These gaseous species evaporate fromthe cores, creating interconnected pore structure within the cores. Theresultant pore structure aids the leaching process in the core removalstep. FIG. 4 shows a SEM micrograph of a transverse view on a polishedsurface showing extensive cracks (˜10 μm in width) leading to the partinterior. The sample was fired up to 1650° C.

FIG. 5 is a XRD spectrum on an as-fired surface of a core fired up to1650° C. in air. It demonstrates that the remaining phases are 3:2mullite and α-Al2O2. FIG. 6 illustrates an increase in the Modulus ofRupture at room temperature (RT MOR) with the addition of carbon fiber.The 4 point bend results are from 5″ long test bars fired up to 1650° C.As illustrated in FIG. 6, the unique microstructure due to transientcarbon fiber reinforcement improves mechanical strength to help coresmaintain structure integrity while achieving higher porosity.

After metal casting, the disposable core die may be removed usingchemical, thermal, mechanical methods or a combination comprising atleast one of the foregoing methods. Examples of such methods includechemical dissolution, chemical degradation or a combination comprisingat least one of the foregoing methods of removing. In the case of aturbine blade, the core may be leached from the cast metal turbine bladeusing a caustic agent that dissolves the ceramic core while notaffecting the cast metal part.

This written description uses examples to disclose the invention,including the preferred embodiments, and also to enable any personskilled in the art to practice the invention, including making and usingany devices or systems and performing any incorporated methods. Thepatentable scope of the invention is defined by the claims, and mayinclude other examples that occur to those skilled in the art. Suchother examples are intended to be within the scope of the claims if theyhave structural elements that do not differ from the literal language ofthe claims, or if they include equivalent structural elements withinsubstantial differences from the literal language of the claims.Aspects from the various embodiments described, as well as other knownequivalents for each such aspect, can be mixed and matched by one ofordinary skill in the art to construct additional embodiments andtechniques in accordance with principles of this application.

What is claimed is:
 1. A method of producing a ceramic core for investment casting, the method comprising at least a step of: injecting a slurry into a disposable die, the slurry comprising ceramic particles, binders, and carbon fibers.
 2. The method of claim 1, wherein at least a portion of the ceramic core defines an internal surface of a turbine blade.
 3. The method of claim 1, wherein the slurry includes carbon fibers in a concentration not exceeding 20 wt % of the slurry.
 4. The method of claim 1, wherein the carbon fibers have an average diameter of 200 microns or less.
 5. The method of claim 4, wherein the carbon fibers have an average diameter of 100 microns or less.
 6. The method of claim 1, wherein the carbon fibers have an aspect ratio of greater than 1:1 up to 100:1.
 7. The method of claim 6, wherein the carbon fibers have an aspect ratio of greater than 10:1 up to 100:1.
 8. The method of claim 1, further comprising at least one additional heating step that removes the disposable die.
 9. The method of claim 1, further comprising at least one additional heating step that substantially removes the carbon fibers.
 10. A fired ceramic core comprising ceramic particles and fiber-shaped voids, the fiber-shaped voids generally aligned with an axis of the core.
 11. The ceramic core of claim 10, wherein at least a portion of the ceramic core defines an internal surface of a turbine blade.
 12. The ceramic core of claim 10, wherein the voids have an average diameter of 200 microns or less.
 13. The ceramic core of claim 10, wherein the voids have an average diameter of 100 microns or less.
 14. The ceramic core of claim 10, wherein the voids have an aspect ratio of greater than 1:1 up to 100:1.
 15. The ceramic core of claim 10, wherein the voids have an aspect ratio of greater than 10:1 up to 100:1.
 16. A cured ceramic core comprising ceramic particles, a binder, and carbon fibers, the carbon fibers generally aligned with an axis of the core.
 17. A method of producing a ceramic core for investment casting, the method comprising at least a step of: heating a disposable die, the disposable die comprising a cured composition of ceramic particles, binders, and carbon fibers, the heating oxidizing and substantially removing the carbon fibers leaving voids. 